The accurate measurement of humidity is crucial to a number of diverse fields, including meteorology, materials processing and manufacturing and environmental control (HVAC). These applications require measurement over a very wide range of water vapor concentrations (greater than 1% to less than 1 part per million by volume), over a large range of ambient temperatures (170 degrees K. to 500 degrees K.). Presently, instruments which accurately measure water vapor concentration typically use optical techniques consisting of optical absorption and optical detection of dew point. Of these techniques, optical dew point detection is normally favored in cases where extreme accuracy and reproducability are required. For example, so called "chilled mirror" dew point hygrometers are often used as reference sensors for calibration. Unfortunately, these instruments are large, complex and expensive. This precludes their use in a large number of important applications. As an alternative, many approaches for microsensor-based humidity measurement have been explored. These include a variety of solid state sensors which generally measure the effect of water on the electrical properties of some material. These microsensors have a number of drawbacks, including poor sensitivity, large hysteresis, limited operating range and significant aging effects.
Accordingly, a miniature hygrometer, with the inherent accuracy and reproducability of optical dew point hygrometers is needed for a number of applications. Such sensors must be able to measure water vapor over a wide range of concentrations in widely varying ambient environments. Past efforts of developing solid state microsensors have not been entirely successful. Typically, these sensors require exotic solid state materials in order to achieve the required sensitivity.
Specifically, the normal electronic transduction mechanisms are not sensitive enough for efficient operation. In order to overcome this, most microsensors use a variety of hygroscopic materials to increase water uptake in the sensor. This approach is unattractive because it leads to a number of errors, including non-linearity, hysteresis, temperature sensitivity and aging effects. As a result, these miniature sensors are not suitable for situations in which high accuracy and reproducability are needed in diverse operating conditions.
Dew point detection provides a way of accurately measuring water vapor content. Unfortunately, current dew point sensors employ inefficient transducers for water vapor detection. Such devices use a chilled mirror in conjunction with an optical reflectance transducer to measure the condensation of moisture at the dew point temperature. The optical transducers presently used are both bulky and slow. In addition, they require relatively complex electronics. The result is that current dew point hygrometers are large, slow and complicated devices. The performance of current dew point sensors also suffers from the fact that they are sensitive to contamination (e.g., dust loading of the mirror surface). This limitation relates to the actual measurement algorithm used in the device: the moisture transducer's surface temperature is feedback controlled to stay at dewpoint. Because of this, unwanted shifts in moisture transducer output due to its inherent instabilities lead to errors in measured dewpoint. Presently, this feedback technique is used primarily to offset the limited speed of the sensor. Improving the speed of the moisture transducer and reducing its size would allow a new measurement algorithm to be used. A new approach is therefore needed to allow miniaturization of its device.
One dew point detection device that is relevant to the present invention is described in U.S. Pat. No. 4,378,168 issued Mar. 29, 1983 to Kuisma et al. This prior art dew point detection device comprises a piezo-electric sensor having a material for transmitting acoustic waves along a surface that is subjected to the condensation and presence of dew and liquid to be detected. A wave inducing device includes a transmitter for producing an acoustic surface wave on the surface in conjunction with piezo-electric phenomena of the sensor. A detector includes a receiver for receiving the wave after transmission thereof, across the surface of the device. The wave is variably attenuated in transmission between the transmitter and the receiver in accordance with the dew or liquid on the surface and a measurement device measures the attenuation of the detected wave and thereby indicates the dew point or presence on the surface. Unfortunately, the method and apparatus disclosed in the Kuisma et al patent, does not address a major problem in current instruments, namely contamination-based errors. More specifically, in the apparatus described in the aforementioned prior art patent, the stability of the surface acoustic wave dew point sensor is required for accurate measurement. Such stability is required because a control loop is being used to hold a parameter of the surface acoustic wave sensor, such as attenuation or impedence, fixed. Because of this, drifts in the dew sensor will lead to errors in dew point. Thus, despite the disclosure in the Kuisma et al patent, the need for a miniaturized hygrometer, capable of measuring dew point with extreme accuracy, remains an unfilled need.